1. Field of the Invention
This invention pertains to microelectromechanical devices and circuits, and more particularly, to formation of such devices within and upon an integrated package.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for providing miniaturized sensors and actuators and performing functions not done or poorly done by semiconductor integrated circuit (IC) technology. As compared to transistor switches formed with conventional IC technology, for example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance. Although the fabrication steps used to form MEMS devices are similar to those for forming integrated circuits, packaging of MEMS devices presents some additional complexity. Because MEMS devices tend to have moving parts, they cannot be encapsulated in the manner used for protection of purely electronic circuits. Instead, a hermetically sealed enclosure, or xe2x80x9ccavityxe2x80x9d, around the MEMS device typically needs to be formed. The final step in fabrication of the MEMS device itself is typically a xe2x80x9creleasexe2x80x9d of the device, in which a sacrificial layer is removed so that the device may move freely. The release process may be quite critical, involving, for example, careful anneals, etching and drying processes. After release of the MEMS device, any processing steps which may contaminate the device must be avoided until the protective enclosure around the device is formed.
As is done in IC manufacturing, many MEMS devices (or circuits) are typically formed on a single substrate, which is subsequently diced, or singulated, to separate the individual devices. The dicing is typically done using a saw, and is a particularly xe2x80x9cdirtyxe2x80x9d and mechanically stressful process. Therefore it is preferable that MEMS have enclosures formed over them if they are released before dicing (also referred to as being released at xe2x80x9cwafer levelxe2x80x9d). Alternatively, the devices may be singulated first and then released before additional packaging is done. In either case, the individual MEMS die are typically put into packages fairly similar to IC packages. The packaging could involve, for example, attaching the back side of the MEMS device substrate to the top side of a packaging substrate, then wire-bonding contact pads on the top side of the MEMS device substrate to contact pads on the packaging substrate, and affixing a cap to form the sealed cavity over the MEMS device (unless the device was already covered prior to singulation). This type of packaging scheme has disadvantages, however. For example, the individual handling of each die needed to place it into a package is expensive and potentially unreliable. The package size also has to be relatively large in order to accommodate the MEMS chip substrate and the wire bonds. Furthermore, the use of wire bonding can limit the performance of high-frequency devices by introducing parasitic impedances.
One alternative approach, also used in IC packaging, is to eliminate wire bonds through flip-chip bonding with solder bumps or balls. If the solder bumps are large enough, a MEMS chip can be flip-chip bonded to a packaging substrate with enough clearance for the (now upside-down) MEMS device to operate. An enclosure can be formed around the MEMS device using the packaging substrate, MEMS substrate, and an additional underfill material applied laterally around the device, as described in U.S. Pat. No. 6,214,644. Although this flip-chip approach may improve the reliability and performance of the package connections by removing the wire bonds, it is still necessary to handle individual MEMS die one at a time during packaging, and to perform multiple packaging steps (e.g. forming solder bumps, flip-chip bonding, underfill application and cure).
It would therefore be desirable to develop a MEMS packaging method and structure which reduces the need for individual device handling, improves package size and cost, and improves high-frequency performance.
The problems outlined above may be in large part addressed by formation of a MEMS device upon a packaging substrate, rather than upon a separate device substrate. A packaging substrate as used herein is a substrate similar to those used in packaging of integrated circuits. A packaging substrate is formed from an insulating material and includes conductive features on its lower surface. The conductive features, which may include conductive pads, pins, bumps or balls, are adapted for use in electrical coupling of the substrate to a circuit board or other circuit carrier. xe2x80x9cCircuit boardxe2x80x9d as used herein may refer to a circuit board, carrier, or other surface to which a miniature circuit may be mounted. The packaging substrate may also include conductive interconnects extending within and through the substrate, where the interconnects are adapted to connect the conductive features to the upper surface of the substrate. The xe2x80x9cupper surfacexe2x80x9d of the substrate as used herein is the surface upon which an IC would be mounted if the substrate were used in an IC-mounting context, while the xe2x80x9clower surfacexe2x80x9d is the surface that would face the circuit board in such a context. When a MEMS device is formed upon a packaging substrate, the underside of at least one element of the device is in contact with the upper surface of the substrate. This is in contrast to formation of the MEMS device upon a separate MEMS substrate, which might then be mounted upon the packaging substrate (either die-up mounting which would typically include wire bonds, or flip-chip, die-down mounting). In an embodiment, the underside of the MEMS device element may be formed upon an exposed end of a conductive interconnect within the substrate, allowing electrical coupling of the device element to the lower surface of the substrate or to another device or circuit formed on the substrate. Alternatively, the device element may be connected to an interconnect within the substrate through an interconnect formed on the surface of the substrate.
An embodiment of a microelectromechanical circuit as contemplated herein may further include a cover spaced above the device and the substrate. The cover may be spaced sufficiently above the device to permit proper electromechanical operation of the device. The circuit may further include a sealing structure interposed between the substrate and cover, where the sealing structure laterally surrounds the device. The sealing structure may include, for example, an adhesive or a metal layer. In an embodiment, the substrate, cover and sealing structure combine to form a protective enclosure around the device. In an additional embodiment of the microelectronic circuit, an IC may be mounted on the packaging substrate in a position laterally spaced from that of the MEMS device. The IC may be electrically coupled to the MEMS device by wire bonds or through interconnects within the substrate, and may be included within a protective enclosure formed around the device. In some embodiments, the IC may alternatively be external to the protective enclosure.
Some embodiments of the circuit described herein are believed to provide cost, manufacturability, and/or performance advantages. For example, multiple MEMS devices may be formed simultaneously upon a packaging substrate. In a preferred embodiment, the devices may be released and then covered before dicing. At the time of dicing, the MEMS devices are already packaged and protected. The devices may therefore be ready for assembly to a circuit board or carrier, with no further packaging required. Costly and unreliable handling of individual devices during packaging may therefore be avoided. Fabrication of the packaged MEMS device is believed to be relatively simple and inexpensive in some embodiments, since after formation and release of the device, the only remaining steps may be affixing a cover and dicing. Affixing of the cover may be done individually on each of the multiple devices, or at the xe2x80x9cwafer levelxe2x80x9d on the devices as a group. High-frequency performance of the MEMS device may also be improved in some embodiments, since wire bonds and bulky packages are replaced with the low-loss, low-inductance conductive interconnects within the package substrate. The package size may also be reduced, since clearance is not needed for mounting of an additional MEMS substrate or for wire bonds.
In addition to the microelectromechanical circuit described above, an array of microelectromechanical circuits is contemplated herein. In an embodiment, the array includes first and second microelectromechanical devices formed laterally spaced upon the upper surface of a packaging substrate, first and second covers spaced above the substrate and the first and second devices, respectively, and a sealing structure interposed between the substrate and the first and second covers, where the sealing structure laterally surrounds each of the first and second devices. In an embodiment, the first and second covers may be portions of a single cover for the array. The substrate, sealing structure, and first and second covers may combine to form a protective enclosure for each of the first and second devices. In such an embodiment, the protective enclosures are adapted to remain intact after separation of the substrate portions underlying the first and second devices (singulation of the devices). In a further embodiment, the first and second devices are adapted to be operable through application of electrical signals to appropriate conductive features on the lower surface of the substrate, either before or after singulation.
A method of forming a microelectromechanical device includes forming the device upon a packaging substrate having one or more conductive features upon its lower surface, where an underside of at least one element of the device is in contact with the upper surface of the substrate. Forming the device may include patterning a conductive layer deposited upon the packaging substrate, and may further include releasing the device to permit electromechanical operation. The method may further include affixing a cover to the substrate, where the cover is spaced above the device. Affixing the cover may include interposing a sealing structure between the substrate and the cover, and the substrate, cover and sealing structure may combine to form a protective enclosure around the device. In such an embodiment, the method may also include simultaneously forming an additional enclosed device laterally spaced upon the substrate, where the additional device is covered by an additional cover and laterally surrounded by an additional sealing structure. The additional cover and the cover may in an embodiment be portions of a single larger cap, and the additional sealing structure and the sealing structure may be portions of a single larger sealing layout. The method may further include separating the device from the additional device, where the device and the additional device each remains protectively enclosed after the separation.